#include <stdio.h>

int main( )
{
    int n, i;
    double c[ 5000 ], A0, An1, sum;
    while ( scanf("%d", &n) != EOF )
    {
        scanf("%lf%lf", &A0, &An1);
        sum = 0;
        for ( i = 1; i <= n; i++ )
        {
            scanf("%lf", &c[ i ]);
            sum += 2 * ( n + 1 - i ) * c[ i ];
        }
        printf("%.2lf\n", ( An1 + n * A0 - sum ) / ( n + 1 ));
    }
    return 0;
}
